The present invention relates to DC-DC power converters, and, more specifically, to an active snubber circuit, method of operation thereof and power converter employing the same. The present invention is directed, in general, to pulse width modulated DC-DC power converters which process power from an input DC voltage source and deliver power to a load through an inductive energy storage component being alternatively connected to the input DC power source and to the load via electronic solid state switches.
It is known from the art that increasing the operational frequency of DC-DC power converters results in reduction of total weight, size and cost as well as in increase of converted power density, i.e. number of watts per cubic inch.
However, the solid-state switches of the DC-DC power converters are subjected to high power losses as a result of the switch being changed from one state to another (i.e. when the switch turns on or off) while having simultaneously overlapped both a significant current through it and significant (exceeding the primary source level) voltage across it. This results in extensive heat dissipation within the switch during the switching transitions.
For xe2x80x9coff-onxe2x80x9d transition the switching losses may be defined as:
Won=0,5VsmIsmton;xe2x80x83xe2x80x83[1]
for xe2x80x9con-offxe2x80x9d transition the switching losses may be defined as:
Woff=0,5VsmIsmtoff;xe2x80x83xe2x80x83[2]
where: Vsm is a voltage-across maximum value during the transition,
Ism is a current-through maximum value during the transition,
ton is a time-duration of the xe2x80x9coff-onxe2x80x9d transition,
toff is a time-duration of the xe2x80x9con-offxe2x80x9d transition.
The switching transition losses evolve substantial constraints upon the potentially available performance rate of existing DC-DC power converters wherein the bipolar junction transistors (BJT), insulated gate bipolar transistors (IGBT) and metal-oxide-semiconductor field-effect transistors (MOSFET) are used as power switches.
Fast switching speeds, low power gate drive and state-on low resistance of MOSFETs have made them a wide practical choice. However, the MOSFETs exhibit large drain-to-source capacitance Coss. It reduces the dV/dt factor on turnoff and minimizes the power loss at this transition but increases the power loss at tum-on transition since the power stored in Coss is fully dissipated as heat within the MOSFET, which may be defined as:
Pon=0,5CossV2smfop,xe2x80x83xe2x80x83[3]
where: fop is an operational frequency value.
To reduce the switching transition power losses within the DC-DC power converters, the prior art brought forward numerous passive, i.e. comprising the inductive and capacitive components only, and active, i.e. comprising solid-state semiconductor devices, snubber circuits optimally shaping the operating points trajectories of the switching devices, i.e. adjusting the shape-of-change of the voltage-across and of the current-through to minimize their simultaneous overlapping during the switching transition.
Passive snubbers are hardly attractive since the power absorbed within their passive components is dissipated as heat. Active snubbers are more efficient since the absorbed power may be re-circulated back to the primary source or forwarded to the load.
Shaping the operating points trajectories of the power switches becomes extremely important function with increasing the operational frequency, operational voltages and overall power conversion output.
As well as the power switches of the DC-DC power converters, the switching devices within prior art active snubbers are also subjected to power losses ascribed with [1] and [2].
Minimizing these xe2x80x9csnubberxe2x80x9d losses is no less important function both for high and low rates of power conversion since in the latter case the xe2x80x9csnubberxe2x80x9d losses may be in the row with power conversion output.
Therefore, a better method and apparatus for active shaping the operating points trajectories both of the power switches within the DC-DC power converters and of the switching devices within the apparatus itself is needed to be applicable for use in various DC-DC power converter topologies.
The benefits of the proposed invention may be better disclosed through prior appraisal of the state-of-the-art snubber circuits.
Although the present invention may be applicable equally to many existing DC-DC power converter topologies, the boost converter topology is chosen as an example to demonstrate the advantages of the present invention.
The output voltage of the boost converter is always higher than the peak value of the mains voltage, and would be typically between 300 and 400 volts. At these high voltage levels the switching transition losses are unavoidably great, and transient voltage and current spikes may well damage the solid-state semiconductor devices. For this reason a fast-recovery blocking rectifier is required. At a high operational frequency, a fast-recovery rectifier is subjected to substantial reverse-recovery current and, therefore, produces significant reverse-recovery loss when operated under a xe2x80x9chard switchingxe2x80x9d condition, i.e. when simultaneous overlapping of non-zero-voltage-across with nonzero-current-through during the switching transition.
Besides, as being galvanically non-isolated of the primary power source, the boost converters are quite sensitive to reverse-recovery sufficiency to prevent the internal components of electric shoot-through destruction. As a result, the xe2x80x9chard-switchedxe2x80x9d DC-DC power converters are operated at relatively low switching frequencies.
To reduce the switching transition losses while increasing the switching frequency and, therefore, to improve the efficiency of DC-DC power conversion, a number of xe2x80x9csoft-switchingxe2x80x9d techniques have been proposed within the prior art.
xe2x80x9cSoft-switchingxe2x80x9d condition occurs when no voltage appears across the switch and/or no current flows through the switch during the switching transition.
Turning the power switch into conducting state at zero voltage across it (ZVS=Zero Voltage Switching) results in elimination of two kinds of switching transition losses: the first, caused by blocking rectifier reverse-recovery loss as defined in [2] and, the second, caused by the power switch stray capacitance recharge as defined in [3].
Turning the power switch into nonconducting state at zero current through it (ZCS=Zero Current Switching) results in elimination of inductively stored power loss which may be defined as:
Poff=0,5LIsmfop,xe2x80x83xe2x80x83[4]
where: L is an inductance value of the power storage inductor.
FIG. 1 illustrates the circuit diagrams of DC-DC power converters comprising some of the prior art snubber circuitry for soft-switching conditions provision and for switching transition loss reduction, and indexed structures are as follows:
200: prior art snubber;
209: controllable commutator;
210: controllable commutating switch within the controllable commutator 209;
211: rectifier within the first commutator 209;
213: commutating rectifier;
215: damp rectifier;
216: resonant inductor;
LH: auxiliary saturable inductor;
217: first slope-shaping capacitor;
218: second slope-shaping capacitor;
304: main power switch;
305: power storage inductor;
306: blocking rectifier;
307: output smoothing filter;
308: primary power source;
309: load.
These prior art techniques utilize an auxiliary active commutator 209 together with a few passive components like resonant inductor 216 and voltage slope-shaping capacitors 217, 218 thus forming an active snubber to limit the rate-of-change of blocking rectifier 306 current (di306/dt) and to create the soft-switching conditions for the main power switch 304. As a result the main power switch 304 is tuned-on into conducting state under zero-voltage across it. However, the auxiliary active commutator 209 shown in FIG. 1(a) operates under hard-switching condition since it is turned-off into non-conducting state while carrying a current greater than the input current, and subsequently turned-on into conducting state while the voltage across it is equal to the output voltage. Since the peak resonant current within the resonant inductor 216 may be twice greater as within the power storage 305 to satisfy the zero-voltage soft-switching condition for the main power switch 304, then turning-off the auxiliary active commutator 209 into non-conducting state is accomplished with considerable power loss.
To reduce the switching loss within the auxiliary active commutator 209 the snubber circuits shown in FIGS. 1(b, c, d, e) have been proposed in the prior art.
However, the next drawback of the prior art techniques is undesirable resonance between the output capacitance Coss of the auxiliary active commutator 209 and the resonant inductor 216. Attempting to eliminate this resonance by including an auxiliary saturable inductor LH, as shown in FIG. 1(a) increases the number of coil components and the overall cost. Besides, due to undesirable circulation of magnetically stored energy within the resonant inductor 216, it is a source of transient EMI noise, and also produces additional power loss. To eliminate undesirable power circulation the snubber circuits shown in FIGS. 1(b, c) include auxiliary components increasing the overall complexity and associated power loss.
The common drawback of the described above prior art circuitry is evolved by undesirable resonant circulation of energy between the resonant inductor 216 and multiple parasitic capacitance within the snubber circuit 200, which also produces power loss accomplished with voltage spikes and EMI radiation affecting the electronic equipment. The finite time of resonant circulation results in limiting the power conversion operational duty factor and in limiting the opportunity to increase the power conversion operational frequency.
Besides, the prior art circuitry shown in FIG. 1 cannot be incorporated into the isolated DC-DC power converter topologies.
Other prior art techniques require too complex control circuitry which are sensitive to transient noise.
Therefore, what is needed in the art is a circuit that eliminates the above described drawbacks.
The purpose of this invention is to reduce the switching transition losses of power within switching devices of the switching type pulse-width-modulated DC-DC power converters of various topologies, therefore to create a family of modified converters of a higher efficiency than the prior art by including the active soft-switching conditioner into conventional converter structures.
The advantages of the proposed invention are that through active shaping the operating points trajectories of the switching devices both within the DC-DC power converter structure and within the active soft-switching conditioner the following benefits are achieved:
providing soft-switching zero-voltage-across/zero-current-through conditions within the time intervals of alternative changing between conducting and non-conducting states both for power switching devices of the converter and for networks commutating devices of the active soft-switching conditioner;
eliminating the switching transition power losses resulted from simultaneous overlapping non-zero-voltage-across/non-zero-current-through conditions during switching transitions within the power switching devices and within the networks commutating devices;
reducing the conduction power losses within the power switching devices;
increasing the power conversion operational frequency increase and hence the power storing components decrease in weight and volume;
improving the dynamic controllability of the converter and the power conversion process regulation quality;
reducing the radiated EMI;
re-circulating the energy absorbed by the active soft-switching conditioner back to the primary power source or forwarding it to the load of the converter.
According to the invention, the modified converters comprise at least; an input means to be connected to the primary power source; an output means to be connected to the load;
a common return bus to be connected between the primary power source and the load;
a power storage inductor to accumulate the power absorbed from the primary power source and to deliver the power to the load;
a controllable power switch operated in a pulse-width-modulated fashion and alternatively turned into conducting state to provide the power absorption from the primary power source into the power storage inductor and turned into nonconducting state to provide the power release from the power storage inductor into the load;
a power rectifier to disconnect the load from the power storage inductor and from the primary power source while the controllable power switch is conducting and to provide the power release path from the power storage inductor and from the primary power source to the load while the controllable power switch is non-conducting;
an output smoothing filter to store the power delivered to the load and to absorb the ripple component of delivered power;
a active soft-switching conditioner connected through its nodes across the controllable power switch to provide active shaping the operating points trajectories of the switching devices through active developing soft-switching zero-voltage-across/zero-current-through conditions within the time intervals of alternative changing between conducting and nonconducting states.
The active soft-switching conditioner comprises at least:
an input node, an output node, a common node;
a separator comprising at least a rectifier to separate the networks within the active soft-switching conditioner;
first commutator comprising a controllable switch connected in parallel with a rectifier to provide first controllable path for currents within the network of the active soft-switching conditioner;
second commutator comprising at least a rectifier to provide second controllable path for currents within the network of the active soft-switching conditioner;
third commutator to provide third controllable path for currents within the network of the active soft-switching conditioner;
fourth commutator comprising a rectifier or, according to the embodiment of active soft-switching conditioner, a controllable switch connected in parallel with a rectifier to provide fourth controllable path for currents within the network of the active soft-switching conditioner;
first slope-shaper comprising at least a capacitor to provide shaping the voltage wave form developed across the controllable power switch during its transition into non-conducting state, therefore creating soft-switching zero-voltage-across condition for the controllable power switch during its transition into non-conducting state such that the controllable power switch transition into non-conducting state does not produce power loss;
second slope-shaper comprising at least one capacitor to provide shaping the voltage wave form developed across the first commutator, therefore creating soft-switching zero-voltage-across/zero-current-through conditions during the first commutator transition into non-conducting state such that the first commutator transition into nonconducting state does not produce power loss;
damp/resonant choke to provide the prescribed rate-of-change for the current through the power rectifier during its transition into non-conducting state, therefore creating soft-switching close to zero-current-through condition for the power rectifier during its transition into nonconducting state such that the power rectifier transition into non-conducting state does not produce power loss, and to provide the resonant discharge path for the capacitor within the first slope-shaper for shaping the voltage wave form developed across the controllable power switch during its transition into conducting state, therefore creating soft-switching zero-voltage-across/zero-current-through condition for the controllable power switch during its transition into conducting state such that the controllable power switch transition into conducting state does not produce power loss;
damp switch comprising at least a rectifier or a controllable switch connected in parallel with a rectifier to provide a current path to release the energy magnetically stored within the damp/resonant choke, and to damp the parasitic circulation of energy magnetically stored within the damp/resonant choke;
the first slope-shaper is connected between the input node and the common node to shunt the controllable power switch;
A controllable switch within the first commutator is turned into conducting state prior to the controllable power switch transition into conducting state to provide the prescribed rate-of-change for the current through the power rectifier during its transition into non-conducting state, therefore creating soft-switching close to zero-current-through condition for the power rectifier during its transition into non-conducting state such that the power rectifier transition into non-conducting state does not produce power loss, and to provide the resonant discharge path for the capacitor within the first slope-shaper, therefore creating soft-switching zero-voltage-across/zero-current-through condition for the controllable power switch during its transition into conducting state such that the controllable power switch transition into conducting state does not produce power loss;
Past the controllable switch within the first commutator is turned into non-conducting state the second slope-shaper is connected across the damp/resonant choke to shunt it and to absorb its released energy, therefore to provide the prescribed shape-of-change for the voltage across the first commutator during its transition into non-conducting state, therefore creating soft-switching zero-voltage-across/zero-current-through condition for the first commutator such that the first commutator transition into non-conducting state does not produce power loss;
the third commutator connected with the second slope-shaper is adapted to limit the voltage level across the second slope-shaper during resonant release of energy magnetically stored within the damp/resonant choke, and to provide a discharge path for the capacitor(s) within the second slope-shaper past the controllable power switch transition into non-conducting state, and to provide a prescribed rate-of-change for the voltage across the controllable power switch during its transition into non-conducting state, therefore creating soft-switching zero-voltage-across condition for the controllable power switch such that its transition into non-conducting state does not produce power loss.
The separator in conjunction with third commutator and fourth commutator provides an opportunity to re-circulate the energy magnetically absorbed into the damp/resonant choke both to the primary power source or to the load by cutting the absorbed energy release path off the controllable power switch and off the power storage inductor by turning the fourth commutator into non-conducting state, therefore enhancing the functional applicability of the active soft-switching conditioner towards greater number of different converter topologies.
Past the controllable power switch transition into non-conducting state, first slope-shaper by charging its internal capacitor, and second slope-shaper by discharging its internal capacitor(s) to the primary power source or to the load, both shunt the controllable power switch therefore diverting its current and providing the soft-switching zero-voltage-across conditions to eliminate the switching transition power loss within the controllable power switch. When the controllable power switch is performed as a solid-state semiconductor device then its stray capacitor may be used for the first slope-shaper.
The damp switch commutates the damp/resonant choke with magnetically stored energy release circuit such that the damp/resonant choke is disconnected from the magnetically stored energy release circuit as soon as the current through the damp/resonant choke reaches close to zero, therefore eliminating the parasitic circulation of energy magnetically stored within the damp/resonant choke, hence improving the dynamic controllability of the converter and the power conversion process regulation quality, and reducing the radiated EMI.
The controllable power switch and all controllable switches within the damp switch, within the first commutator and within the fourth commutator may be performed as solid-state semiconductor switches.
The body diodes of the solid-state semiconductor switches may be used as the rectifiers connected across the controllable switch within the damp switch, across the controllable switch within the first commutator and across the controllable switch within the fourth commutator.
The high-power pulse diodes may be used as the power rectifier, the separator and the third commutator.
The further purpose of the proposed invention is the method to reduce the power losses within the switching devices of the power converters comprising the discussed active soft-switching conditioner, including the following steps:
turning simultaneously into conducting state both the controllable switch within the first commutator and the controllable switch within the fourth commutator prior to the controllable power switch transition into conducting state to provide the prescribed rate-of-change for the current through the power rectifier during its transition into non-conducting state, therefore creating soft-switching close to zero-current-through condition for the power rectifier such that its transition into non-conducting state does not produce power loss, and to provide the resonant discharge path for the capacitor within the first slope-shaper, therefore creating soft-switching zero-voltage-across/zero-current-through condition for the controllable power switch such that its transition into conducting state does not produce power loss;
turning the power rectifier into non-conducting state with soft recovery of its reverse resistance under soft-switching close to zero-current-through condition hence losslessly disconnecting the load from the power storage inductor and from the primary power source;
connecting the power storage inductor to the primary power source for power absorption and accumulation through the networks within the active soft-switching conditioner;
turning the controllable power switch into conducting state under soft-switching zero-voltage-across/zero-current-through conditions hence connecting the power storage inductor to the primary power source for power absorption and accumulation;
shunting the damp/resonant choke with a series-connection network comprising the second slope-shaper connected with the damp switch to provide the prescribed shape-of-change for the voltage across the first commutator as soon as its transition into non-conducting state starts and for the voltage across the fourth commutator as soon as its transition into non-conducting state starts, therefore creating soft-switching zero-voltage-across/zero-current-through conditions for the first commutator transition into non-conducting state and for the fourth commutator transition into non-conducting state such that the first commutator transition into non-conducting state and the fourth commutator transition into nonconducting state do not produce power losses;
releasing the energy magnetically stored within the damp/resonant choke through the forward-biased second commutator, through the forward-biased separator and through the forward-biased damp switch into the capacitors within the second slope-shaper and further into the output node as soon as past a prescribed time the third commutator becomes forward-biased/conducting;
turning the damp switch into non-conducting state as soon as its carried current decreases close to zero and soft-switching zero-current-through condition occurs, therefore eliminating the parasitic circulation of energy magnetically stored within the damp/resonant choke;
recovering the second commutator reverse resistance under soft-switching close to zero-current-through condition;
absorbing the power from the primary power source into the power storage inductor through the controllable power switch;
turning the controllable power switch into non-conducting state under soft-switching zero-voltage-across condition;
absorbing the power from the primary power source into the power storage inductor through linear charging the first slope-shaper and through linear discharging the second slope-shaper;
connecting the primary power source and the power storage inductor to the load as a result of the power rectifier transition into conducting state and forwarding the absorbed and accumulated power from the power storage inductor to the load and to the output smoothing filter.
According to the method and apparatus disclosed, the further advantages may therefore outflow evolving the following opportunities:
provision of soft-switching zero-voltage-across/zero-current-through conditions within the time intervals of alternative changing between conducting and non-conducting states both for power switching devices of the DC-DC power converter and for networks commutating devices of the active soft-switching conditioner;
elimination of the switching transition power losses resulted from simultaneous overlapping of non-zero-voltage-across/non-zero-current-through conditions during switching transitions within the power switching devices and within the networks commutating devices;
reduction of the conduction power losses within the power switching devices;
increase of the power conversion operational frequency and hence decrease in weight and size of the power storing components;
improvement of the dynamic controllability of the DC-DC power converter and of the power conversion process regulation quality;
reduction of the radiated EMI;
re-circulation of the energy absorbed by the active soft-switching conditioner back to the primary power source or forward to the load of the converter;
applicability for use in various DC-DC power converter topologies;
applicability for use in isolated DC-DC power converter topologies.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.